The present invention relates to a fault detection system for a continuously running transfer press of the type which includes a workpiece transfer system. The fault detection system of this invention automatically protects both the transfer system and the transfer press from damage in the event the transfer system fails to maintain synchronization with the transfer press.
For many years transfer systems have been used to automatically move a workpiece through a series of die stations to form both metal and nonmetal workpieces to desired shapes. In such forming operations, workpieces are formed in a series of stages by a series of mating dies. Typically, the movable dies are mounted to a reciprocating press slide or ram. Such reciprocating slides may be driven mechanically, hydraulically, pneumatically, or by various high energy techniques, such as gas combustion. The combination of a transfer system and a press is known in the industry typically as a transfer press.
Workpiece transfer systems have been used in the past with each of the types of presses mentioned above. In many applications mechanically-driven, continuously running presses provide important advantages. Mechanically-driven presses often provide particularly high production rates when used to produce metal parts.
In the past, workpiece transfer systems for use on mechanically-driven, continuous presses have typically been mechanically driven from the press drive through a series of gears and shafts, cog belts and pulleys, or chains and sprockets. Because such workpiece transfer feeds are mechanically driven from the press drive, synchronization of the transfer system with the reciprocating die mounted to the slide is automatically and precisely maintained.
The function of the transfer system is to clamp or grip the workpiece in each die with clamping fingers as the die is opening, lift the workpiece out of the lower die (on workpieces that are not lifted out of the die completely by lifting devices built into the die), transfer the workpiece to the next die, lower the workpiece onto the die, and then unclamp the fingers before the dies close. If the clamping fingers do not retract at the proper time, the clamping fingers can be crushed between the dies, thereby damaging the die, the transfer system, and the workpieces being formed.
The production rates obtainable with transfer presses are typically limited by the speed at which workpieces may be transferred without misfeeds, or by the velocity of the moving die which may be used without tearing, rupturing, or cracking the workpiece being formed. The size of workpieces produced in transfer presses has increased greatly in recent years. Similarly, the size of transfer presses has also increased, both in forming capacity and in physical dimensions. This increase in the size of parts formed in a transfer press has required an increase in the length of the clamp, lift and transfer strokes of the workpiece transfer system. Typically, each of these three motions (clamp, lift and transfer) is cyclical, stopping at the end of each stroke, dwelling, and then returning. Thus, the motions of a transfer system can be considered as intermittent, and they place fluctuating torque loads on the transfer drive. The recent increase in production rate, part size, and physical size of transfer presses has severely taxed the limits of mechanical transfer drives.
Furthermore, presses are generally started and stopped by large scale dry friction clutches and brakes. Engagement of such clutches tends to start the press abruptly, and engagement of the brake (preceded by disengagement of the clutch) stops the press abruptly. This abrupt starting and stopping of the press can have detrimental effects on mechanical transfer drive systems.
Electric motors and fluid motors and sophisticated electronic controls have been used for many industrial applications in the past. For example, coil feeding equipment has been used to feed coil stock into metal-forming presses by means of electric motors for many years. A failure in any component in the electronic control causing malfunction in the coil feed cycle in such a system causes little or no damage except to some formed parts. This is because no part of the feeding equipment extends under the moving die.
In addition, single cycle presses have been used with electric motor-driven transfer systems. The single cycle mode ensures that the transfer system is withdrawn from under the die properly before die closure is initiated. In this case, the press is started only after the transfer system has withdrawn to a safe position. The press makes one stroke and then stops with the slide in the top stroke position, at which time the unloading transfer system is moved into the die area from the unload side of the press in order to pick up the workpiece and remove the formed workpiece from the die. As the unloading transfer system is moving the formed workpiece out of the die, a sensor is tripped to initiate a loading transfer system to move the next workpiece to be formed into the die. As the loading transfer system retracts from the area of the moving die, a sensor is tripped to start the next press cycle.
In this prior art approach, any malfunction of the unloading or loading transfer systems (which prevents either of them from completing their full cycle and tripping the appropriate sensor) simply stops the production cycle without causing damage to the die or the press. Of course, such single stroking or single cycling of a press and transfer system severely inhibits the maximum production rate obtainable.